jlaitine commented on code in PR #16194:
URL: https://github.com/apache/nuttx/pull/16194#discussion_r2059065297
##########
sched/semaphore/sem_trywait.c:
##########
@@ -74,29 +77,57 @@ int nxsem_trywait_slow(FAR sem_t *sem)
/* If the semaphore is available, give it to the requesting task */
- semcount = atomic_read(NXSEM_COUNT(sem));
+ if (mutex)
+ {
+ new = nxsched_gettid();
+ }
+
+ lock = atomic_read(NXSEM_COUNT(sem));
do
{
- if (semcount <= 0)
+ if (mutex)
{
- leave_critical_section(flags);
- return -EAGAIN;
+ if (NXSEM_MACQUIRED(lock))
+ {
+ goto out;
+ }
+ }
+ else
+ {
+ if (lock <= 0)
+ {
+ goto out;
+ }
+
+ new = lock - 1;
}
}
- while (!atomic_try_cmpxchg_acquire(NXSEM_COUNT(sem),
- &semcount, semcount - 1));
+ while (!atomic_try_cmpxchg_acquire(plock, &lock, new));
/* It is, let the task take the semaphore */
ret = nxsem_protect_wait(sem);
if (ret < 0)
{
- atomic_fetch_add(NXSEM_COUNT(sem), 1);
+ if (mutex)
+ {
+ atomic_set(NXSEM_MHOLDER(sem), NXSEM_NO_MHOLDER);
Review Comment:
Yes, It would require:
1) priority protection enabled on the mutex
2) someone, "X", on higher priority blocks on the mutex after it is acquired
in the above loop, but before this call protect_wait
3) execution returns here while "X" is blocked.
4) protect_wait fails
I believe each step is possible - this needs to be fixed.
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